A flat panel light source system is based on the generation of photo-luminescence within a light cavity structure. Optical power is delivered to the light emissive pixels in a controlled fashion through the use of optical waveguides and a novel addressing scheme employing Micro-Electro-Mechanical Systems (MEMS) devices. The energy efficiency of the light source results from employing efficient, innovative photo-luminescent species in the emissive pixels and from an optical cavity architecture, which enhances the excitation processes operating inside the pixel. The present system is thin, light weight, power efficient and cost competitive to produce when compared to existing technologies. Further advantages realized by the present system include high brightness in varying lighting conditions; high color gamut; viewing angle independence, size scalability without brightness and color quality sacrifice, rugged solid-state construction, vibration insensitivity and size independence. The present invention has potential applications in military, personal computing and digital HDTV systems, multi-media, medical and broadband imaging displays and large-screen display systems. Defense applications may range from full-color, high-resolution, see-through binocular displays to 60-inch diagonal digital command center displays. The new light source system employs the physical phenomena of photo-luminescence in a flat-panel light source system.
Conventional transmissive liquid crystal displays (LCDs) use a white backlight, together with patterned color filter arrays (CFAs), to create colored pixel elements as a means of displaying color. Polarizing films polarize light. The pixels in a conventional liquid crystal display are turned on or off through the use of an additional layer of liquid crystals in combination with two crossed polarizer structures on opposite sides of a layer of polarizing liquid crystals. When placed in an electrical field with a first orientation, the additional liquid crystals do not alter the light polarization. When the electrical field is changed to a second orientation, the additional liquid crystals alter the light polarization. When light from the polarizing liquid crystals is oriented at ninety degrees to the orientation of the polarizing film in a first orientation, no light passes through the display, hence, creating a dark spot. In a second orientation, the liquid crystals do rotate the light polarization; hence, light passes through the crystals and polarizing structures to create a bright spot having a color as determined by the color filter array.
This conventional design for creating a display suffers from the need to use a polarizing film to create polarized light. Approximately one half of the light is lost from the backlight; thus, reducing power efficiency. Just as significantly, imperfect polarization provided by the polarizing film reduces the contrast of the display. Moreover, the required additional use of a color filter array to provide colored light from a white light source further reduces power efficiency. If each color filter for a tri-color red, green, and blue display passes one third of the white light, then two thirds of the white light is lost. Therefore, at least 84% of the white light generated by a backlight is lost.
The use of organic light emitting diodes (OLEDs) to provide a backlight to a liquid crystal display is known. For example, U.S. Patent Application Publication No. 2002/0085143 A1, by Jeong Hyun Kim et al., published Jul. 4, 2002, titled “Liquid Crystal Display Device And Method For Fabricating The Same,” describes a liquid crystal display (LCD) device, including a first substrate and a second substrate; an organic light emitting element formed by interposing a first insulating layer on an outer surface of the first substrate; a second insulating layer and a protective layer formed in order over an entire surface of the organic light emitting element; a thin film transistor formed on the first substrate; a passivation layer formed over an entire surface of the first substrate including the thin film transistor; a pixel electrode formed on the passivation layer to be connected to the thin film transistor; a common electrode formed on the second substrate; and a liquid crystal layer formed between the first substrate and the second substrate.
A method for fabricating the LCD in U.S. Patent Application Publication No. 2002/0085143 A1 includes the steps of forming a first insulating layer on an outer surface of a first substrate; forming an organic light emitting element on the first insulating layer; forming a second insulating layer over an entire surface of the organic light emitting element; forming a protective layer on the second insulating layer; forming a thin film transistor on the first substrate; forming a passivation layer over an entire surface of the first substrate including the thin film transistor; forming a pixel electrode on the passivation layer; and forming a liquid crystal layer between the first substrate and a second substrate. However, this prior art design does not disclose a means to increase the efficiency of the LCD.
U.S. Pat. No. 6,485,884 issued Nov. 26, 2002 to Martin B. Wolk et al., titled “Method For Patterning Oriented Materials For Organic Electronic Displays And Devices” discloses the use of patterned polarized light emitters as a means to improve the efficiency of a display. The method includes selective thermal transfer of an oriented, electronically active, or emissive material from a thermal donor sheet to a receptor. The method can be used to make organic electroluminescent devices and displays that emit polarized light. There remains a problem, however, in that there continues to exist incomplete orientation of the electronically active or emissive material from a thermal donor sheet to a receptor. Hence, the polarization of the emitted light is not strictly linearly polarized, therefore, the light is incompletely polarized.
There is a need, therefore, for an alternative backlight design that improves the efficiency of polarized light production, thus and thereby improving the overall efficiency of a liquid crystal display that incorporates the alternative backlight.
Stereoscopic displays are also known in the art. These displays may be formed using a number of techniques; including barrier screens such as discussed by Montgomery in U.S. Pat. No. 6,459,532 and optical elements such as lenticular lenses as discussed by Tutt et al in U.S. Patent Application Publication No. 2002/0075566. Each of these techniques concentrates the light from the display into a narrow viewing angle, providing an auto-stereoscopic image. Unfortunately, these techniques typically reduce the perceived spatial resolution of the display since half of the columns in the display are used to display an image to either the right or left eye. These displays also reduce the viewing angle of the display, reducing the ability for multiple users to share and discuss the stereoscopic image that is being shown on the display.
Among the most commercially successful stereoscopic displays to date have been displays that either employed some method of shuttering light such that the light from one frame of data is able to enter only the left or right eye and left and right eye images are shown in rapid succession. Two methods have been employed in this domain; including displays that employ active shutter glasses or passive polarizing glasses. Systems employing shutter glasses display either a right or left eye image while an observer wears active LCD shutters that allow the light from the display to pass to only the appropriate eye. While this technique has the advantage that it allows a user to see the full resolution of the display and allow the user to switch from a monoscopic to a stereoscopic viewing mode, the update rate of the display is typically on the order of 120 Hz, providing a 60 Hz image to each eye. At this relatively low refresh rate, most observers will experience flicker resulting in significant discomfort if the display is used for more than a few minutes within a single viewing session. Even when the display is refreshed at significantly higher rates, flicker is often visible when the display is large and/or high in luminance.
Byatt, 1981 (U.S. Pat. No. 4,281,341) has described a system employing a switchable polarizer that is placed in front of a CRT and performs very similarly to shutter glasses, using the polarization to select which eye will see each image. This system has the advantage over shutter glasses that the user does not need to wear active glasses, but otherwise suffers from the same deficiencies, including flicker.
Lipton, 1985 (U.S. Pat. No. 4,523,226) described a display system that will not suffer from flicker, but instead uses two separate video displays and optics to present the images from the two screens appropriately for the two eyes. While this display system does not suffer from the same visual artifacts as the system employing switchable polarization that was described by Byatt, the system requires two separate visual displays and additional optics, providing increasing the cost of such a system.
Previously, Newsome disclosed the use of upconverting phosphors and optical matrix addressing scheme to produce a visible display in U.S. Pat. No. 6,028,977. Upconverting phosphors are excited by infrared light; this method of visible light generation is typically less efficient than downconversion (luminescent) methods like direct fluorescence or phosphorescence, to produce visible light. The present invention differs from the prior art in that a different addressing scheme is employed to activate light emission from a particular emissive pixel. The method and device disclosed herein do not require that two optical waveguides intersect at each light emissive pixel. Furthermore, novel optical cavity structures, in the form of asymmetrical light emitting resonators, are disclosed for the emissive pixels in the present invention.
Additionally, in U.S. Patent Application Publication No. US2002/0003928A1, Bischel et al. discloses a number of structures for coupling light from the optical waveguide to a radiating pixel element. The use of reflective structures to redirect some of the excitation energy to the emissive medium is disclosed. In the present invention, we disclose the use of novel asymmetrical optical cavity structures, in the form of elliptical ring, so-called racetrack, or elliptical disk resonators, the resonators themselves modified to affect the emission of visible light.
Recently, the optical properties of asymmetrical microdisk resonators have been disclosed in “Highly Directional Emission From Few-Micron-Size Elliptical Microdisks”, Applied Physics Letters, 84, 6, ppg. 861-863 (2004), by Sun-Kyung Kim, et al. Such asymmetrical structures exhibit polarized light emission with the axis of polarization parallel to the major axis of the elliptical structure. The use of such asymmetrical structures to produce polarized light sources is a novel feature of the present invention.
The use of such resonators further allows for a novel method of control of the emission intensity, through the use of Micro-Electro-Mechanical Systems (MEMS) devices to alter the degree of power coupling between the light power delivering waveguide and the emissive resonator pixel. Such means have been disclosed in control of the power coupling to opto-electronic filters for telecommunications applications. In this case, the control function is used to tune the filter. Control over the power coupling is described in “A MEMS-Actuated Tunable Microdisk Resonator”, by Ming-Chang M. Lee and Ming C. Wu, paper MC3, 2003 IEEE/LEOS International Conference on Optical MEMS, 18-21 August 2003.